Signal transmission over a wireless channel suffers from multipath propagation which causes symbols transmitted at different time instants to interfere with each other at a receiver, a phenomenon known as intersymbol interference (ISI). Such multipath ISI channels induce frequency-selective fading which can attenuate different frequencies of the transmit signal differently. Receiver techniques to mitigate the effects of frequency-selectivity so as to restore the originally transmitted signal are widely referred to as equalization. When the channel is frequency non-selective (i.e. frequency-flat), no ISI is experienced and equalization is accomplished by simple gain-control, adjusting the magnitude and the phase of the received signal.
An OFDM signal consists of multiple narrowband subcarriers that are mutually orthogonal to one another. This special structure of OFDM converts a frequency-selective channel into multiple parallel narrowband frequency-flat channels each of which affords a detection based on simple gain-control, obviating the need for complex equalization circuitry that would be required by single-carrier modulations.
When the individual narrowband signals combine, the OFDM waveform exhibits high peak-to-average-power-ratio (PAPR), a common figure that describes the magnitude variation of waveform samples. A high PAPR dictates the use of a linear transmit chain to avoid signal distortion that results in degraded error performance and spectral re-growth beyond the intended signal bandwidth. In particular, power amplifier (PA) characteristics exhibit a saturation of output power with increased input power—a nonlinear behavior. To ensure linearity with a high PAPR, the PA is operated away from saturation, i.e. with a power back-off. Operating with a power back-off results in decreased transmission range and reduced power efficiency; the latter defined to be the ratio of the output power to the input source power of the amplifier. When a large transmit power is required, the over-provisioning of the PA may render OFDM transmission infeasible. A multitude of OFDM PAPR reduction methods exist that generally offer rules for modifying the input or the output of the OFDM modulator to enforce tolerable levels of signal peak occurrence. Many of these rules alter the fundamental frequency-domain input-output relationship of the OFDM.